Method for creating S/TEM sample and sample structure

ABSTRACT

An improved method and apparatus for S/TEM sample preparation and analysis. Preferred embodiments of the present invention provide improved methods for TEM sample creation, especially for small geometry (&lt;100 nm thick) TEM lamellae. A novel sample structure and a novel use of a milling pattern allow the creation of S/TEM samples as thin as 50 nm without significant bowing or warping. Preferred embodiments of the present invention provide methods to partially or fully automate TEM sample creation, to make the process of creating and analyzing TEM samples less labor intensive, and to increase throughput and reproducibility of TEM analysis.

The present application claims priority from PCT Application No.PCT/US2007/082163, filed Oct. 22, 2007, and from U.S. Prov. Pat. App.No. 60/853,183, filed Oct. 20, 2006, which are hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to preparation of samples and methods ofanalysis for transmission electron microscopes and scanning transmissionelectron microscopes.

BACKGROUND OF THE INVENTION

Semiconductor manufacturing, such as the fabrication of integratedcircuits, typically entails the use of photolithography. A semiconductorsubstrate on which circuits are being formed, usually a silicon wafer,is coated with a material, such as a photoresist, that changessolubility when exposed to radiation. A lithography tool, such as a maskor reticle, positioned between the radiation source and thesemiconductor substrate casts a shadow to control which areas of thesubstrate are exposed to the radiation. After the exposure, thephotoresist is removed from either the exposed or the unexposed areas,leaving a patterned layer of photoresist on the wafer that protectsparts of the wafer during a subsequent etching or diffusion process.

The photolithography process allows multiple integrated circuit devicesor electromechanical devices, often referred to as “chips,” to be formedon each wafer. The wafer is then cut up into individual dies, eachincluding a single integrated circuit device or electromechanicaldevice. Ultimately, these dies are subjected to additional operationsand packaged into individual integrated circuit chips orelectromechanical devices.

During the manufacturing process, variations in exposure and focusrequire that the patterns developed by lithographic processes becontinually monitored or measured to determine if the dimensions of thepatterns are within acceptable ranges. The importance of suchmonitoring, often referred to as process control, increases considerablyas pattern sizes become smaller, especially as minimum feature sizesapproach the limits of resolution available by the lithographic process.In order to achieve ever-higher device density, smaller and smallerfeature sizes are required. This may include the width and spacing ofinterconnecting lines, spacing and diameter of contact holes, and thesurface geometry such as corners and edges of various features. Featureson the wafer are three-dimensional structures and a completecharacterization must describe not just a surface dimension, such as thetop width of a line or trench, but a complete three-dimensional profileof the feature. Process engineers must be able to accurately measure thecritical dimensions (CD) of such surface features to fine tune thefabrication process and assure a desired device geometry is obtained.

Typically, CD measurements are made using instruments such as a scanningelectron microscope (SEM). In a scanning electron microscope (SEM), aprimary electron beam is focused to a fine spot that scans the surfaceto be observed. Secondary electrons are emitted from the surface as itis impacted by the primary beam. The secondary electrons are detected,and an image is formed, with the brightness at each point of the imagebeing determined by the number of secondary electrons detected when thebeam impacts a corresponding spot on the surface. As features continueto get smaller and smaller, however, there comes a point where thefeatures to be measured are too small for the resolution provided by anordinary SEM.

Transmission electron microscopes (TEMs) allow observers to seeextremely small features, on the order of nanometers. In contrast SEMs,which only image the surface of a material, TEM also allows analysis ofthe internal structure of a sample. In a TEM, a broad beam impacts thesample and electrons that are transmitted through the sample are focusedto form an image of the sample. The sample must be sufficiently thin toallow many of the electrons in the primary beam to travel though thesample and exit on the opposite site. Samples, also referred to aslamellae, are typically less than 100 nm thick.

In a scanning transmission electron microscope (STEM), a primaryelectron beam is focused to a fine spot, and the spot is scanned acrossthe sample surface. Electrons that are transmitted through the workpiece are collected by an electron detector on the far side of thesample, and the intensity of each point on the image corresponds to thenumber of electrons collected as the primary beam impacts acorresponding point on the surface.

Because a sample must be very thin for viewing with transmissionelectron microscopy (whether TEM or STEM), preparation of the sample canbe delicate, time-consuming work. The term “TEM” as used herein refersto a TEM or an STEM and references to preparing a sample for a TEM areto be understood to also include preparing a sample for viewing on anSTEM. The term “S/TEM” as used herein also refers to both TEM and STEM.

Several techniques are known for preparing TEM specimens. Thesetechniques may involve cleaving, chemical polishing, mechanicalpolishing, or broad beam low energy ion milling, or combining one ormore of the above. The disadvantage to these techniques is that they arenot site-specific and often require that the starting material besectioned into smaller and smaller pieces, thereby destroying much ofthe original sample.

Other techniques generally referred to as “lift-out” techniques usefocused ion beams to cut the sample from a substrate or bulk samplewithout destroying or damaging surrounding parts of the substrate. Suchtechniques are useful in analyzing the results of processes used in thefabrication of integrated circuits, as well as materials general to thephysical or biological sciences. These techniques can be used to analyzesamples in any orientation (e.g., either in cross-section or in planview). Some techniques extract a sample sufficiently thin for usedirectly in a TEM; other techniques extract a “chunk” or large samplethat requires additional thinning before observation. In addition, these“lift-out” specimens may also be directly analyzed by other analyticaltools, other than TEM. Techniques where the sample is extracted from thesubstrate within the FIB system vacuum chamber are commonly referred toas “in-situ” techniques; sample removal outside the vacuum chamber (aswhen the entire wafer is transferred to another tool for sample removal)are call “ex-situ” techniques.

Samples which are sufficiently thinned prior to extraction are oftentransferred to and mounted on a metallic grid covered with a thinelectron transparent film for viewing. FIG. 1A shows a sample mountedonto a prior art TEM grid 10. A typical TEM grid 10 is made of copper,nickel, or gold. Although dimensions can vary, a typical grid mighthave, for example, a diameter of 3.05 mm and have a middle portion 12consisting of cells 14 of size 90×90 μm² and bars 13 with a width of 35μm. The electrons in an impinging electron beam will be able to passthrough the cells 14, but will be blocked by the bars 13. The middleportion 12 is surrounded by an edge portion 16. The width of the edgeportion is 0.225 mm. The edge portion 16 has no cells, with theexception of the orientation mark 18. The thickness 15 of the thinelectron transparent support film is uniform across the entire samplecarrier, with a value of approximately 20 nm. TEM specimens to beanalyzed are placed or mounted within cells 14.

For example, in one commonly used ex-situ sample preparation technique,a protective layer 22 of a material such as tungsten is deposited overthe area of interest on a sample surface 21 as shown in FIG. 2 usingelectron beam or ion beam deposition. Next, as shown in FIGS. 3-4, afocused ion beam using a high beam current with a correspondingly largebeam size is used to mill large amounts of material away from the frontand back portion of the region of interest. The remaining materialbetween the two milled rectangles 24 and 25 forming a thin verticalsample section 20 that includes an area of interest. The trench 25milled on the back side of the region of interest is smaller than thefront trench 24. The smaller back trench is primarily to save time, butthe smaller trench also prevents the finished sample from falling overflat into larger milled trenches which may make it difficult to removethe specimen during the micromanipulation operation.

As shown in FIG. 5, once the specimen reaches the desired thickness, thestage is tilted and a U-shaped cut 26 is made at an angle partiallyalong the perimeter of the sample section 20, leaving the sample hangingby tabs 28 at either side at the top of the sample. The small tabs 28allow the least amount of material to be milled free after the sample iscompletely FIB polished, reducing the possibility of redepositionartifacts accumulating on the thin specimen. The sample section is thenfurther thinned using progressively finer beam sizes. Finally, the tabs28 are cut to completely free the thinned lamella 27. Once the finaltabs of material are cut free lamella 27 may be observed to move or fallover slightly in the trench. A completed and separated lamella 27 isshown in FIG. 6.

The wafer containing the completed lamella 27 is then removed from theFIB and placed under an optical microscope equipped with amicromanipulator. A probe attached to the micromanipulator is positionedover the lamella and carefully lowered to contact it. Electrostaticforces will attract lamella 27 to the probe tip 29 as shown in FIG. 7.The tip 29 with attached lamella is then typically moved to a TEM grid10 as shown in FIG. 8 and lowered until lamella is placed on the grid inone of the cells 14 between bars 13.

Whichever method is used, the preparation of sample for TEM analysis isdifficult and time consuming. Many of the steps involved in TEM samplepreparation and analysis must be performed using instruments operatedmanually. For this reason, successful TEM sample preparation generallyrequires the use of highly trained and experienced operators andtechnicians. Even then, it is very difficult to meet any reasonablestandards of reproducibility and throughput.

Use of FIB methods in sample preparation has reduced the time requiredto prepare samples for TEM analysis down to only a few hours. However,CD metrology often requires multiple samples from different locations ona wafer to sufficiently characterize and qualify a specific process. Insome circumstances, for example, it will be desirable to analyze from 15to 50 TEM samples from a given wafer. When so many samples must beextracted and measured, using known methods the total time to processthe samples from one wafer can be days or even weeks. Even though theinformation that can be discovered by TEM analysis can be very valuable,the entire process of creating and measuring TEM samples hashistorically been so labor intensive and time consuming that it has notbeen practical to use this type of analysis for manufacturing processcontrol.

Further, existing TEM sample structures are not robust enough to surviveautomated extraction and mounting. Additionally, TEM structures createdusing prior art methods often suffer from bending or bowing when thinnedto 100 nm or below. In the prior art, manual thinning of TEM sample ishalted when bowing observed by the operator. Such manually observationwould not be desirable in an automated system.

What is needed is a method to more completely automate the TEM sampleextraction and measurement and to increase throughput andreproducibility so that TEM measurement can be incorporated intointegrated or in situ metrology for process control. What is also neededis a method of creating a TEM sample that will not suffer from bowingphenomenon when thinned to 100 nm or less and that is robust enough tosurvive automated extraction and mounting.

SUMMARY OF THE INVENTION

An object of the invention, therefore, is to provide an improved methodfor TEM sample analysis. Preferred embodiments of the present inventionprovide improved methods for TEM sample creation, especially for smallgeometry (<100 nm thick) TEM lamellae. Some preferred embodiments of thepresent invention provide methods to partially or fully automate TEMsample creation, to make the process of creating and analyzing TEMsamples less labor intensive, and to increase throughput andreproducibility of TEM analysis.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a typical prior art TEM grid.

FIGS. 2-5 illustrate the steps in an ex-situ sample preparationtechnique according to the prior art.

FIG. 6 is a micrograph of a completed and separated lamella according tothe prior art.

FIGS. 7-8 illustrate the transfer of a lamella using a probe andelectrostatic attraction according to the prior art.

FIG. 9 is a flowchart showing the steps of creating one or more lamellaeaccording to a preferred embodiment of the present invention.

FIG. 10 shows a lamella site according to the process of FIG. 11 afterhigh precision fiducials have been milled and a protective layerdeposited over the lamella location.

FIG. 11 shows a lamella site according to the process of FIG. 11 afterlow precision fiducials have been milled.

FIG. 12 shows a lamella site according to the process of FIG. 11 afterbulk milling has been completed.

FIG. 13 shows a high resolution micrograph of a lamella sample accordingto the present invention after bulk milling has been completed.

FIG. 14 shows a lamella created according to the process of FIG. 11.

FIG. 15 shows a lamella created according to the process of FIG. 11.

FIG. 16 shows a high resolution micrograph of a lamella according to thepresent invention.

FIG. 17A shows a graphical representation of a dual beam system whereone beam is used to thin the lamella while the other beam images thelamella to endpoint milling.

FIG. 17B shows a graphical representation of a single beam system wherethe sample must be rotated to allow one beam to mill and image forendpointing.

FIG. 17C shows a lamella site during the milling process which could beimaged and the image processed according to the present invention toendpoint milling.

FIG. 18 show a graphical illustration of a milling pattern according tothe present invention that is used to thin the TEM sample

FIGS. 19A through 19C show steps in the milling process of FIG. 18 on across section view of a TEM sample.

FIG. 20 is a high resolution micrograph of a lamella sample with athinner central “window” prepared according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention provide improved methodsfor lamella creation. A preferred embodiment can create S/TEM sampleswith a thickness in the 50-100 nm range for the purposes of S/TEMmetrology with minimal site-to-site variation. The process can produce a10 μm wide×˜5 μm deep×˜500 nm thick lamella with a final-thinned windowof 3 μm×3 μm at the targeted final thickness (50-100 nm). S/TEM samplesproduced according to the present invention will not suffer from bowingphenomenon when thinned to 100 nm or less and are robust enough tosurvive automated extraction and mounting.

A preferred method or apparatus of the present invention has many novelaspects, and because the invention can be embodied in different methodsor apparatuses for different purposes, not every aspect need be presentin every embodiment. Moreover, many of the aspects of the describedembodiments may be separately patentable.

Current TEM lamella creation processes for FIB systems use manual inputas the primary method for locating a feature or site of interest forlamella creation. Typically, once the desired lamella location ismanually located, a fiducial or locating mark is milled nearby. BecauseFIB imaging necessarily causes some sample damage, a protective layer isdeposited over the desired lamella location before imaging and/ormilling. The protective layer makes it harder to see features on thesubstrate so a fiducial mark is typically milled into the protectivelayer to help orient the beam and locate the proper place for a cut.This fiducial is used in subsequent processing as a locating mark. Imagerecognition keyed to this fiducial is then used to find the locationsfor subsequent milling of the lamella. In order to mill the fiducial, alocation near the desired lamella site is typically selected manually,and the desired fiducial pattern is then automatically milled at thatlocation.

This method of manually identifying the lamella site and then manuallyselecting the fiducial location does not provide a high degree ofprecision or accuracy. As a result, known automatic lamella millingroutines are limited to rough milling of lamellae which areapproximately 500 nm thick. Further thinning is typically manuallycontrolled in order to achieve the desired lamella thicknesses of 100 nmor less.

FIG. 9 is a flowchart showing the steps of creating one or more lamellaeaccording to a preferred embodiment of the present invention. In thisembodiment, machine-vision based metrology and image recognition,high-precision fiducial marks, and automatic fiducial placement are usedto significantly improve lamella placement accuracy and precision.Various steps in the process are shown in FIGS. 10 through 16.

First, in step 401, a wafer is loaded into a FIB system, such as aCertus Dual Beam System, commercially available from FEI Company ofHillsboro, Oreg., the assignee of the present invention. In step 402,lamella sites on the wafer surface are located automatically using imagerecognition software. Suitable image recognition software is available,for example, from Cognex Corporation of Natick, Mass. Image recognitionsoftware can be “trained” to locate the desired lamella locations byusing sample images of similar features or by using geometricinformation from CAD data. Automated FIB or SEM metrology can also beused to identify or help identify the lamella site. Metrology mayconsist of image-based pattern recognition, edge finding, ADR,center-of-mass calculations, blobs, etc.

In optional step 404, the lamella site is given a protective 5 kV FIBtungsten deposition 15 μm wide by 3 μm tall for 1:20. This providessufficient tungsten on the site surface to prevent damage during the 30kV FIB site alignment and deposition steps. This protective layer may bedirectly placed if the 5 kV 180 pA FIB aperture to SEM coincidence isless than 4 μm, otherwise a process of site alignment may be used torefine placement of this deposition.

In step 406, the precise locations of any desired fiducial marks withrespect to each desired lamella location are specified. For example,using a FIB or SEM to image a sample location, a fiducial location couldbe specified by an operator using a mouse to drag a virtual box aroundthe desired fiducial location. Automated metrology software could thenprecisely measure the location of the fiducial with respect toidentifiable features at the sample location (for example 15 nm from theright edge of the feature). When each lamella site is located, afiducial can then be automatically milled at each lamella site at theprecise location specified so that the spatial relationship between eachfiducial and each lamella location will be identical. A fiduciallocation could also be specified using CAD data to specify the locationof the fiducial with respect to a particular structure on the wafersurface.

In a preferred embodiment, precise fiducial placement is accomplishedthrough the use of the IC3DTM™ software's vision tools. A specifiedpattern can be located by image recognition software and used to locatea target structure. A series of calipers—a pattern recognition tool thatlocates edges—are then used to find the edges of the target structureand to precisely center the fine fiducials around the target structure.Extensive use of IC3D's shape linking capabilities allows robustplacement of site fiducials based on direct measurement of each site.

Preferably, a combination of high precision (fine) fiducials and lowprecision (bulk) fiducials are used to optimize lamella placementprecision and accuracy. Currently, fiducials used for lamella locationand milling consist only of low-precision features such as an “X” formedby the intersection of two milled lines. At the resolutions necessaryfor adequate lamella production, however, each milled line will beseveral nanometers wide. Edge detection software must be used todetermine the centerline of each milled line and then the intersectionof the two mathematically determined centerlines used to determine aparticular reference point. There is typically too much error in thistype of determination to use the fiducial to accurately determine alamella location within the margin of error needed for manysmall-geometry lamella applications.

In a preferred embodiment, a combination of typical low-precisionfiducial marks and higher precision marks are used. High-precisionfiducials, such as the rectangles 506 shown in FIG. 10 allow the lamellalocation to be much more accurately determined. The rectangularfiducials 506 shown in FIG. 10 are located at either end of the desiredlamella location. High-precision fiducial are smaller than thelow-precision fiducials discussed below. For this reason, thehigh-precision fiducials are not identifiable with the large FIB beamsused for bulk milling, and are only used for final placement of thelamella with smaller FIB beams. The rectangular fiducials in FIG. 10 arelocated using image analysis to determine the Y position of their topand bottom edges. This results accurate positioning even when thefiducial is damaged during FIB imaging. Edge detection software only hasto identify the top and bottom edges to precisely locate the top andbottom edges of the lamella. Pattern recognition for these rectangularfiducials is based on a two-measurement strategy—the top and bottomedges of the fiducial are measured. Once the edge positions are located,a central line or axis can be determined which is parallel to the topand bottom edges of the lamella. As the sample is imaged with the FIB,the top surface is progressively sputtered away. The high precisionfiducial described above is very tolerant of this FIB damage becauseboth measured edges will be altered at nearly the same rate, so theoverall error in lamella placement will be very low.

Low-precision fiducials, such as the large circles 504 in FIG. 11, canbe used for gross-structure pattern recognition, such as quickly findingthe approximate lamella location and performing the bulk milling.Suitable low-precision fiducials can be easily identified when thesample is imaged with a low resolution (higher beam size) ion beamsuitable for rapid bulk material removal. Multiple fiducials andcombinations of low and high precision fiducials and different fiducialshapes (as shown in FIG. 11) can be used for even more accurateorientation.

Once the fiducial locations have been determined, in step 408, highprecision fiducials are milled at the desired locations. As shown inFIG. 10, a small rectangular feature 506 is milled at each end of thelamella site (which is indicated by dashed line 507) with the 1nA 30 kVFIB for vertical placement of the lamella during the final thinningprocess. In a preferred embodiment, a suitable fiducial pattern willallow the final lamella placement to be accurate within 10 nm. In someembodiments, the size and shape of the fiducial can be varied dependingon the size, width, or location of the desired lamella.

In step 410, after the high precision fiducials have been milled, a bulkprotective layer 508 composed of, for example, tungsten or platinum isdeposited over the lamella site to protect the sample from damage duringthe milling process. FIG. 11 shows a lamella site 502 with a protectivelayer 508 deposited over the desired lamella location on a wafer surface503. For some samples where information is required very close to thesurface, it may be useful to deposit the protective layer using a lowenergy FIB (˜5 keV) to perform the deposition. The high precisionfiducials 506 are also preferably lightly backfilled with the protectivematerial to protect them during future processing.

In step 412, after the bulk protective deposition, large circularfiducials 504 as shown in FIG. 11 are milled around the fine fiducials.These low-precision fiducials are used for gross-structure patternrecognition, such as quickly re-finding the approximate lamella locationand determining the location for bulk milling of the lamella. Because alarger beam size will be used for the bulk milling, a suitable lowprecision fiducial should be easily identified by pattern recognitionsoftware even in lower resolution images. The system can then readilyrelocate each desired lamella site by locating the fiducial and knowingthat the lamella site is positioned at a fixed offset from the fiducial.

If there are other lamella sites on the wafer, step 414, the FIB systemnavigates to the coordinates of the next lamella site in step 415. Theprocess then returns to step 402 and steps 402 to 414 are repeated forall remaining lamella sites before the lamella milling process isstarted. Once fiducials have been milled at all lamella sites, in step416, the FIB system navigates to an unmilled lamella site. In step 418,bulk substrate milling is used to roughly shape the lamella. FIG. 12shows a lamella site after the bulk milling of step 418 has beencompleted. A larger ion beam size will be suitable for bulk materialremoval. In a preferred embodiment, each lamella will be formed by usinga FIB to cut two adjacent rectangles 524, 525 on a substrate, theremaining material between the two rectangles forming a thin verticalsample section 527 that includes an area of interest. Preferably, an ionbeam will be directed at the substrate at a substantially normal anglewith respect to the substrate surface. The beam will be scanned in arectangular area adjacent to the sample section to be extracted, thusforming a rectangular hole 524 having a predetermined depth. The milledhole should be sufficiently deep to include the feature of interest inthe extracted sample. Preferably, the milled hole is also deep enough toallow for bulk material to remain at the bottom of the thinned sample(beneath the feature of interest) to increase the mechanical rigidity ofthe sample as discussed below. The beam will be scanned in a rectangulararea 525 adjacent to the sample section to be extracted, but on theopposite side of said sample section from the first rectangular hole.The remaining material between the two rectangular holes will preferablyform a thin vertical sample section that includes the lamella to beextracted.

Low-precision fiducials 504 can be used to control the beam location forbulk milling of the lamella (using a larger beam diameter for more rapidsample removal). A typical cross-section mill pattern can be used comingin from both sides of the lamella, leaving a coarse lamellaapproximately 2 μm thick. The lamella is then further thinned toapproximately 800 nm with a cleaning cross-section mill on both sides inpreparation for the undercut step. FIG. 13 shows a high resolutionmicrograph of a lamella sample after bulk milling has been completed.

In step 420, after the fiducials and bulk mills are done, the lamellaundergoes an undercutting process. The FIB column is preferably tiltedto approximately 4.5 degrees and the lamella bottom undercut with acleaning cross-section at 1nA. Alternatively, the sample stage could betilted. The precise location for the undercut can be located usingvision tools to locate and measure the fine fiducials. Although agreater FIB tilt could be employed (subject to hardware constraints) ashallow incidence angle undercutting provides two benefits to the TEMsample preparation process. First, the lamella face is not imaged at ahigh incidence angle, thus reducing Ga+ implantation and damage; andsecond, the undercutting process serves as an intermediate thinning stepthat has been shown to reduce the lamella thickness to a reasonablynarrow range of widths for a number of different substrates (TI SiGe, TISTI, MetroCal, IFX DTMO, Fujitsu contact). The undercut 602 and sidecuts 604 for a lamella sample 527 are shown in FIG. 14.

In step 422, the sample is then rotated 180 degrees and the processrepeated on the top edge of the lamella in order to cut the bottom free.This results in a rough lamella that is roughly 500 nm thick centeredaround the target structure.

In step 424, two cuts are made from the bottom of the lamella up to nearthe top surface in order to cut the sides of the lamella free, butleaving the lamella hanging by a tab 606 (shown in FIG. 14) on eitherside at the top of lamella. Once the final thinning of the lamella hasbeen completed, a probe can be attached to the lamella and the tabs orhinges severed so that the lamella can be extracted. Alternatively, aprobe can be used to break the lamella hinges as described in co-pendingPCT App. No. PCT/US07/82030, filed on Oct. 20, 2007, which is herebyincorporated by reference.

In optional step 426, IC3D vision tools can be used to locate the finefiducials and remove any redeposition from the bulk milling process aswell as the protective tungsten layer deposited during the fiducialmilling process.

The lamella formed by the first two rectangular bulk-milling cuts andthe undercutting will preferably be roughly 500 nm thick. In step 428,the center of the lamella (containing the area of interest) is thinnedfrom both sides, preferably using a 30 pA beam at 1.2 degrees of FIBtilt with the mill pattern described below. As discussed below, thetypical cleaning mill pattern commonly used for lamella milling causesvery thin lamellae (<100 nm) to bend or bow. Applicants have discoveredthat using a mill pattern resulting in multiple passes of the beam onthe sample face prevents the sample from bowing. This mill pattern,along with other embodiments of a method for eliminating lamella bowingduring the thinning process, is discussed in greater detail below.

The final thinning cuts can be placed using calipers (with imagerecognition) to find the lamella edges, with the final lamella thicknessbeing determined by an offset in the milling position from the lamellaface. For example, for each lamella to be extracted from a sample, theexact location of the lamella can be determined from the fiduciallocation. The first cut is milled at half the desired lamella thicknessaway from the center of the desired sample. Viewing the sample from thetop down, using either FIB or SEM imaging, automated metrology softwarecan then measure the edge of the first cut and the fiducial location andprecisely determine the location of the second cut. Using the locationof the high precision fiducials to precisely control beam location, thelamella can then be thinned using a finely focused FIB to a thickness of100 nm or less in a process that is also highly repeatable.

Preferably, real time pattern recognition can be used to position theFIB. A suitable FIB system providing real time pattern recognition andmetrology is the Certus 3D Dual Beam System available from FEI Company,the assignee of the present invention.

In optional step 430, low-kV cleaning is performed on the final thinnedwindow with a 180 pA 5 kV FIB at 4.5 degrees of tilt. Applicants havediscovered that a 10 second cleaning mill on each face of the lamellaproduces a significant improvement in TEM imaging conditions.

If there are other unmilled lamella sites on the wafer, in step 432, theFIB system navigates to the coordinates of the next unmilled lamellasite. The process then returns to step 416 and steps 416 to 432 arerepeated for all remaining unmilled lamella sites.

The final lamella structure produced by the method of discussed inreference to FIG. 9 is shown in FIGS. 14-16. As discussed below, acenter lamella “window” 610 can be thinned to a thickness of 100 nm orless, leaving thicker surrounding material to provide the sample withincreased mechanical strength. Preferably, the center window isapproximately 3 μm wide, 4 μm deep, and 50-70 nm thick. The thickermaterial surrounding window 610, indicated by reference numeral 612 inFIG. 15, also helps prevent the lamella from bowing during the millingprocess. The increased mechanical strength of this “windowed” lamellastructure is also very desirable when using an ex-situ lamellaextraction device as described in co-pending PCT App. No.PCT/US07/82030, filed on Oct. 20, 2007, which is incorporated byreference. FIG. 16 shows a high resolution micrograph of a lamellacreated using the process described above.

In addition to determining mill locations relative to fiducial marks asdiscussed above, the milling process can be endpointed using top downpattern recognition and metrology. In a preferred embodiment, FIBmilling is carried out in a dual beam FIB/SEM system, as shownschematically in FIG. 17A (not to scale) with vertically mounted FIBcolumn 720 used to mill substrate 503 to create lamella 727 and the SEMcolumn 722 used to image the lamella 727 so that automated metrologysoftware can determine whether the lamella has been thinned to thedesired thickness. Alternatively, a dual FIB system could be used withone beam used to mill and the other used to image. As shownschematically in FIG. 17B (not to scale), a system with a single FIBcolumn 720 could also be used and the sample tilted and rotated so thatthe same beam could be used to mill and image (as is known in the priorart). Skilled persons will recognize that there is a danger of damage tothe lamella if a FIB is used to image the sample.

Referring also to FIG. 17C, after the initial bulk mill 724 is completedon one side of the lamella 727, the endpoint of the second bulk mill 725can be controlled by monitoring the width of the lamella in the samefashion that cross-sections for sub-100 nm features are measured by aCD-SEM.

Typically, to measure the width of cross-section of a structure, a SEMis used in conjunction with automatic metrology software. As theelectron beam is scanned across the exposed cross-section, whethersecondary or backscattered detection is employed, there will typicallybe a change in electron intensity at the edges of the structure. Analgorithm is used to assign an edge position based upon the contrast atthe edges of the structure and to determine the distance between thoseedges.

A preferred embodiment of the present invention makes a novelapplication of these known techniques for cross-section metrology. Thefinal lamella position and thickness would be based on a mill and imagetechnique similar to known slice and view techniques where the FIB in adual beam system is used to expose a sample cross section and the SEM isused to image the sample for automated metrology analysis. Imageprocessing tools such as pattern recognition and edge finding tools canthus be used to precisely control lamella thickness. These types ofprior art “slice and view” techniques are described, for example, inU.S. patent application Ser. No. 11/252,115 by Chitturi et al. for“Method Of Measuring Three-Dimensional Surface Roughness Of AStructure,” which is hereby incorporated by reference, and which isassigned to FEI Company, the assignee of the present invention.

Preferably, thinning would first be completed on one side of thelamella. The location of the initial milling would be controlled usingfiducial location or other metrology as discussed above. The samplewould then be imaged from the top down with either a focused ion beam orscanning electron microscope. As with a CD-SEM, when either the ion beamor the electron beam strikes the surface of substrate, secondaryelectrons and backscattered electrons are emitted. Respectively, theseelectrons will be detected by a secondary electron detector orbackscattered electron detector as is known in the art. The analogsignal produced either by secondary electron detector or backscatteredelectron detector is converted into a digital brightness values. As thebeam (either ion or electron) is scanned across the lamella surface,there will be a change in emitted electron intensity at the edges of thestructure. An algorithm is used to assign an edge position based uponthe difference in brightness values or contrast at either of the edgesof the structure and to determine the distance between those edges. Ifanalysis of the image determines that certain specified criteria are notmet (such as, for example, a minimum desired lamella/sample width) thenthe mill and image processing steps are repeated.

Automated cross-section metrology using CD-SEM has long been used todetermine critical dimensions at the sub-100 nm level. As a result, theprocesses involved have been refined to levels of reliability far beyondthose seen with other less common techniques. CD-SEM metrologytechniques can thus provide levels of reliability and repeatability thatare sufficient to allow the use of TEM samples for in-line processcontrol. This type of automated process control has not been practicalin the prior art because of the problems with sample bowing discussedabove. However, by using the combination of the milling algorithm andsample structure described herein, sample bowing at or below 100 nm canbe greatly minimized. This allows the use of automation of theend-pointing process and eliminates the time consuming manual thinningof the prior art, thus enabling higher volume automated lamella creationfor specific structures.

A suitable dual beam FIB/SEM for practicing a preferred embodiment ofthe present invention would be the CLM-3D Dual Beam System availablefrom FEI Company, the assignee of the present invention. Suitablesoftware to implement fully or partially automated image processing,metrology, and machine control according to the present invention shouldprovide pattern recognition and edge detection tools, along with “dowhile” looping capabilities, such as the IC3D™ software also availablefrom FEI Company.

As discussed above, the bending or bowing commonly associated with thin(less than 100 nm thick) TEM samples can be minimized by using a novelmilling pattern to thin the center window of the lamella. Final thinningof lamellae according to the prior art typically makes use of a millingpattern, often referred to as a clean up cut or cleaning cross section,where the ion beam is scanned one line at a time toward a feature ofinterest. With this cutting pattern, the beam executes a set of linecuts in serial mode. The idea is to gradually step the line cuts intothe exposed surface to clean it. All lines are milled consecutively;milling is completed for each line before moving to the next. The beamis then stepped (in the y-direction) toward the desired sample face andthe process is repeated. Milling is completed in one pass, largely toprevent redeposition of sputtered material on the lamella sample face.

Although the pattern may be varied slightly, the beam is essentially incontact with the cut face almost continuously. Although the exactmechanism is unclear, the end result is that the sample will begin tobow or warp away from the beam when the sample gets thinner than about70 nm. Sample warping is a significant problem because accuratemetrology on a warped sample is very difficult. Further, the region ofinterest can be damaged so that it becomes unusable for furtheranalysis.

Referring to FIGS. 18 and 19A through 19C, in a preferred embodiment ofthe present invention, the TEM sample 527 is thinned by using a FIBmilling pattern that repeatedly steps into the sample cut face in they-direction (as shown by arrow 820) with a decreasing scan speed in (andthus a longer/increasing dwell time as the beam steps into the sample).By “decreasing scan speed,” Applicants mean that the time between stepsbecomes longer as the beam approaches the desired sample face (althoughthe speed at which the beam is scanned back and forth in the x-directionmay not change). Further, several passes of the beam are used to reachthe desired mill depth for the sample face. Mill boxes 810 are notintended to show the exact number of steps or the distance betweensteps, but rather the line gradient is intended to illustrate thedecreasing scan speed and the increasing dwell time as the beam movestoward the final sample face. Although similar raster patterns are knownin the prior art, they have typically been used to rapidly mill deepholes in a substrate. Applicants have discovered, however, that thispattern can be used to precisely thin a lamella without causing samplebowing. This type of raster pattern is typically not used for precisemilling of very small structures, primarily because of concerns overredeposition of sputtered material. When used to mill deep holes, theprocess is often stopped to evaluate the milling progress. By using thepattern on an automated tool and letting the milling finish withoutstopping, redeposition is greatly minimized.

By decreasing the scan speed (and thus increasing the dwell time) of theion beam as the beam moves in the y-direction as shown by arrow 820, thetotal beam current is increased for each x-location at each successivebeam step as the beam moves toward the desired sample face 850, 851.This allows for differential milling of the sample, with deeper millingoccurring at the sample face 850. FIGS. 19A to 19C shows a cross-sectionof the sample 527 taken along line 19 in FIG. 18 at various times duringthe milling process. As shown in FIG. 19B, the depth of the milledtrench 840 slopes toward the sample face 850 (getting deeper as thesample face is approached). Milling throughput is thus increased(because areas away from the sample face do not need to be milled asdeeply). The preservation of sample bulk material 860 at the bottom ofthe lamella window 610 also increases the mechanical rigidity of thesample and improves its handling characteristics.

In addition to the varying dwell time or step size, several passes ofthe ion beam are preferably used to reach the desired mill depth for thesample face. Preferably, these multiple beam passes are made withoutchanging the beam angle, energy, current, current density, or diameter.In contrast to the clean up cut described above, the ion beam is notkept in constant contact with the sample face. For example, five or sixpasses could be used to mill final TEM sample 527. Each pass only millsa fraction of the desired depth from initial y-coordinate 822 to finaly-coordinate 824. If the desired sample face depth has not been achievedwhen a pass is completed, the beam is moved away from the sample face(in the y-direction as shown by arrow 821) back to initial y-coordinate822 to begin another pass. Applicants now believe that sample bowing inthe prior art results from either electrostatic forces or thermalbuildup as the beam is in contact with the sample face. By usingmultiple passes so that the beam is not in constant contact with thesample face, any heat/electrostatic charge buildup is allowed todissipate between ion beam passes. As a result, bowing of the final TEMsample 527 is largely eliminated, even for samples as thin as 50 nm.

In many embodiments, all cuts can be performed using a single beamcurrent, for example, a 20 nA beam current using beam energies that arereadily available with many commercial FIB instruments (e.g., 10s ofkeV), with a dwell time and step size that is typically used for FIBmilling of Si. While preferred process parameters are described, skilledpersons will understand that the preferred process parameters will varywith the size and shape of the sample and the material of the substrate.Skilled persons will be able to readily determine suitable processparameters for extracting samples in different applications.

As shown in FIGS. 14-16, the process described above can be used tofurther thin a center lamella “window” 610 to a thickness of 100 nm orless, preferably using the milling pattern discussed above, leavingthicker surrounding material to provide the sample with increasedmechanical strength. Preferably, the center window is approximately 3 μmwide, 4 μm deep, and 50-70 nm thick. The thicker material surroundingwindow 610, indicated by reference numeral 612 in FIG. 15, also helpsprevent the lamella from bowing during the milling process. Theincreased mechanical strength of this “windowed” lamella structure isalso very desirable when using an ex-situ lamella extraction device asdescribed in co-pending PCT App. No. PCT/US07/82030, filed on Oct. 20,2007, which is incorporated by reference. FIG. 20 is a high resolutionmicrograph of a lamella sample with a thinner central “window” preparedaccording to the present invention using the mill pattern describedabove.

Alternatively, in some circumstances it might be desirable to use themill pattern described above to thin the entire lamella to a uniformthickness of 100 nm or less (without a thinner central “window”).Although such a structure would not have the increased mechanicalstrength of a windowed structure (due to the thicker material at thesides and bottom) sample bowing would still be significantly reducedover the prior art.

The present invention provides a number of significant advantages overthe prior art. Using typical methods for TEM sample preparation, ittakes highly trained and experienced operators approximately 3 hours tocreate and extract one sample lamella suitable for TEM analysis. Forcurrent in-line metrology techniques like top-down SEM or CD-SEManalysis, as many as 20 different sites across a wafer might be need tobe measured. Using prior art methods of TEM sample preparation, it wouldtake about 60 hours just to prepare suitable TEM samples from 20different sites.

Also, because so much of the TEM sample preparation must be performedmanually, the process is very labor intensive and requires the use ofhighly skilled operators (which of course translates into high laborcosts).

Further, current manual TEM sample preparation methods produce sampleshaving a great deal of variation. In order to use a metrology techniquefor process control, it is highly desirable that the samples be asuniform as possible. Because current methods require the final thinningof a TEM lamella to be manually controlled, there is an unavoidablevariation in sample thickness for lamellae from different sample sites.Manual control over other key elements in the sample creation process,such as fiducial placement (which determines the actual lamellalocation) introduces even more variation and further reduces theprecision of the final lamella preparation. The variation betweensamples is even greater when lamellae are prepared by differentoperators.

Using the present invention, however, results in a significantimprovement in the TEM sample preparation process. As discussed above,preferred embodiments of the present invention have been used to createand extract S/TEM samples with a thickness in the 50-100 nm range withvery minimal site-to-site variation. The process produces a lamella inroughly 18 minutes, with a site-to-site 3-sigma final lamella thicknessvariation of roughly 20 nm. The time required to sample 20 differentsites on a wafer surface drops to about 6 hours (as opposed to 60 hoursfor current methods). The process is also much less labor intensive anddoes not require operators with as high a degree of training orexperience. Because more of the process is automated, variation betweenlamella samples is also minimized.

The increased throughput and reproducibility of the TEM analysisprovided by the present invention will allow TEM based metrology onobjects such as integrated circuits on semiconductor wafer to be usedfor in-line process control. For example, TEM analysis according to thepresent invention could be utilized in a wafer fabrication facility toprovide rapid feedback to process engineers to troubleshoot or improveprocesses. This kind of process control for the very small features thatcan only be measured by TEM is not possible using prior art TEM samplepreparation methods.

The invention has broad applicability and can provide many benefits asdescribed and shown in the examples above. The embodiments will varygreatly depending upon the specific application, and not everyembodiment will provide all of the benefits and meet all of theobjectives that are achievable by the invention. For example, in apreferred embodiment TEM lamella samples are created using a galliumliquid metal ion source to produce a beam of gallium ions focused to asub-micrometer spot. Such focused ion beam systems are commerciallyavailable, for example, from FEI Company, the assignee of the presentapplication. However, even though much of the previous description isdirected toward the use of FIB milling, the milling beam used to processthe desired TEM samples could comprise, for example, an electron beam, alaser beam, or a focused or shaped ion beam, for example, from a liquidmetal ion source or a plasma ion source, or any other charged particlebeam.

Also, the invention described above could be used with automatic defectreviewing (ADR) techniques, which could identify defects via die-to-dieor cell-to-cell ADR. A lamella containing the defect could be createdand removed with or without milling fiducials. Although much of theprevious description is directed at semiconductor wafers, the inventioncould be applied to any suitable substrate or surface. Further, wheneverthe terms “automatic” “automated” or similar terms are used herein,those terms will be understood to include manual initiation of theautomatic or automated process or step. The accompanying drawings areintended to aid in understanding the present invention and, unlessotherwise indicated, are not drawn to scale.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

We claim as follows:
 1. A method of thinning a sample section for TEManalysis, the method comprising: loading the sample to be thinned intoan ion beam system; thinning the sample section by directing asubstantially normal ion beam at a first side of the sample section in amilling pattern that thins the sample section in a series of passes,each pass having a scan speed and comprising moving the beam in a rasterpattern from the outside of the sample section inward to the desiredsample face and then returning to the outside of the sample section, theseries of passes continuing until the first side of the sample sectionhas been thinned to a desired depth wherein said scan speed decreases asit gets closer to said desired depth; and automatically thinning thesample section by directing a substantially normal ion beam at theopposite second side of the sample section in a milling pattern thatthins the sample section in a series of passes, each pass having a scanspeed and comprising moving the beam in a raster pattern from theoutside of the sample section inward to the desired sample face and thenreturning to the outside of the sample section, the series of passescontinuing until the second side of the sample section has been thinnedto a desired depth wherein said scan speed decreases as it gets closerto said desired depth.
 2. The method of claim 1 wherein moving the beamin a raster pattern from the outside of the sample section inward to thedesired sample face and then returning to the outside of the samplesection comprises moving the beam in a raster pattern having anx-direction parallel to the desired sample face and a y-directionperpendicular to the desired sample face, said raster pattern comprisingscanning the beam back and forth in the x-direction and then steppingthe beam forward toward the desired sample face, said steps continuinguntil the desired sample face is reached.
 3. The method of claim 2wherein the time between forward steps becomes longer as the beamapproaches the desired sample face.
 4. The method of claim 2 wherein thebeam dwell time increases as the beam approaches the desired sampleface.
 5. The method of claim 1 further comprising at the conclusion ofeach pass in the milling pattern moving the ion beam away from thesample face and beginning a new pass.
 6. The method of claim 5 whereinmultiple passes of the beam are used to reach the desired mill depth forthe sample face.
 7. The method of claim 1 wherein either heat orelectrostatic charge buildup is allowed to dissipate between ion beampasses.
 8. The method of claim 6 in which said multiple beam passes aremade without changing the beam angle, energy, current, current density,or diameter.
 9. The method of claim 1 in which thinning the samplesection comprises thinning a central portion of the sample, leavingthicker material at the bottom and sides of the thinned portion.
 10. Themethod of claim 9 in which the thinned central portion is approximately3 μm wide, 4 μm deep, and less than 70 nm thick.
 11. The method of claim1 further comprising imaging the sample section during the thinningprocess and using automatic metrology software to determine whether thedesired sample thickness has been reached.
 12. A system for thinning asample section for TEM analysis, comprising: a sample stage forsupporting the sample section; an ion beam source for producing an ionbeam to mill the sample section; and a controller programmed to controlthe ion beam source and the stage to carry out the method of claim 1.13. The method of claim 1 further comprising extracting the sample froma wafer, wherein loading the sample to be thinned into an ion beamsystem occurs after the sample is extracted from the wafer.
 14. A methodof extracting a microscopic sample from a substrate, the methodcomprising: defining a sample section to be extracted on a substrate;directing a substantially normal ion beam at the substrate surface, saidbeam being scanned in a rectangular area to form a first rectangularhole having a predetermined depth, said first rectangular hole beingadjacent to the sample section to be extracted; directing said beam atthe substrate surface, said beam being scanned in a rectangular area toform a second rectangular hole having a predetermined depth, said secondrectangular hole being adjacent to the sample section to be extractedbut on the opposite side of said sample section from the firstrectangular hole so that the remaining material between the tworectangles forms a thin vertical wafer that includes the sample sectionto be extracted; directing the ion beam at the remaining material at anon-normal angle in order to undercut the remaining material; rotatingthe sample by 180 degrees; directing the ion beam at the remainingmaterial at a non-normal angle from the opposite side of the samplesection in order to free the bottom of the sample section from thesubstrate; directing the ion beam at the remaining material at anon-normal angle in order to free the sides of the sample sectionleaving at tab of material on either side of the sample sectionconnecting the sample section to the substrate; thinning the samplesection by directing a substantially normal ion beam at a first side ofthe sample section in a milling pattern that thins the sample section ina series of passes, each pass having a scan speed and comprising movingthe beam in a raster pattern from the outside of the sample sectioninward to the desired sample face and then returning to the outside ofthe sample section, the series of passes continuing until the first sideof the sample section has been thinned to a desired depth wherein saidscan speed decreases as it gets closer to the desired depth; thinningthe sample section by directing a substantially normal ion beam at theopposite second side of the sample section in a milling pattern thatthins the sample section in a series of passes, each pass having a scanspeed and comprising moving the beam in a raster pattern from theoutside of the sample section inward to the desired sample face and thenreturning to the outside of the sample section, the series of passescontinuing until the second side of the sample section has been thinnedto a desired depth wherein said scan speed decreases as it gets closerto the desired depth; severing the tabs of material on either side ofthe sample section connecting the sample section to the substrate inorder to free the sample; and removing the sample from the substrate.15. The method of claim 14 wherein moving the beam in a raster patternfrom the outside of the sample section inward to the desired sample faceand then returning to the outside of the sample section comprises movingthe beam in a raster pattern having an x-direction parallel to thedesired sample face and a y-direction perpendicular to the desiredsample face, said raster pattern comprising scanning the beam back andforth in the x-direction and then stepping the beam forward toward thedesired sample face, said steps continuing until the desired sample faceis reached.
 16. The method of claim 15 wherein the time between forwardsteps becomes longer as the beam approaches the desired sample face. 17.The method of claim 15 wherein the beam dwell time increases as the beamapproaches the desired sample face.
 18. The method of claim 14 furthercomprising at the conclusion of each pass in the milling pattern movingthe ion beam away from the sample face and beginning a new pass.
 19. Themethod of claim 18 wherein multiple passes of the beam are used to reachthe desired mill depth for the sample face.
 20. The method of claim 14wherein either heat or electrostatic charge buildup is allowed todissipate between ion beam passes.
 21. The method of claim 19 in whichsaid multiple beam passes are made without changing the beam angle,energy, current, current density, or diameter.
 22. The method of claim14 in which thinning the sample section comprises thinning a centralportion of the sample, leaving thicker material at the bottom and sidesof the thinned portion.
 23. The method of claim 22 in which the thinnedcentral portion is approximately 3 μm wide, 4 μm deep, and less than 70nm thick.
 24. The method of claim 14 further comprising imaging thesample section during the thinning process and using automatic metrologysoftware to determine whether the desired sample thickness has beenreached.
 25. The method of claim 14 further comprising: after thinningthe first and second sides of the sample section, imaging the samplesite; using automated metrology software to process the image todetermine the thickness of the thinned sample section; if analysis ofthe image determines that the sample section is thicker than the desiredthickness, redirecting the ion beam at the first and second sides tothin the sample section; and repeating these steps until the desiredthickness is reached.
 26. The method of claim 14 wherein all steps areperformed automatically without human intervention.